Heterogeneous reaction of NO with hematite, goethite and magnetite: Implications for nitrate formation and iron solubility enhancement.

Atmospheric processing may significantly increase solubility of iron in mineral dust, but the effects of heterogeneous reactions on iron solubility have been poorly understood. In this work, we investigated heterogeneous reaction of NO (15 ± 1 and 2.5 ± 0.1 ppmv, equal to ∼3.7 × 10 and ∼6.2 × 10 molecule cm) with hematite, magnetite and goethite at different relative humidities (RH, 0-90%), and changes in particulate nitrate and soluble iron due to heterogeneous reaction with NO were quantified as a function of time (up to 24 h). After reaction with 2.5 ± 0.1 ppmv NO for 24 h (or less time), hematite and magnetite were fully saturated, while goethite was only partly deactivated. Nitrate yield was largest for goethite, and the mass ratio of formed nitrate to unreacted mineral only reached ∼1% or less after 24 h reaction. All the three minerals showed low reactivities towards NO, and the average reactive uptake coefficients of NO in the first 3 h were found to be < 5 × 10. In addition, the increase in iron solubility was found to be small and in some cases even insignificant for the three minerals after heterogeneous reaction with NO for 24 h. Overall, the impacts of heterogeneous reaction of NO with hematite, magnetite and goethite on nitrate aerosol formation and iron solubility could be very limited.